Wildland-Urban Fire Models

Overview of the Wildland-Urban Fire Dynamics Simulator (WFDS)

The Wildland-Urban Interface Fire Dynamics Simulator (WFDS) is an extension of NIST's structural Fire Dynamics Simulator (FDS) to fuels that include vegetation. WFDS uses computational fluid dynamics methods to solve the governing equations for buoyant flow, heat transfer, combustion, and the thermal degradation of vegetative fuels. The solution method makes use of large eddy simulation techniques to solve the gas-phase equations on computational grids that are too coarse to directly resolve the detailed physical phenomena.

WFDS is fully integrated in FDS. However, the most recent implementation of WFDS is always on this webpage. The version of WFDS on the FDS webpage may somewhat lag behind the version found here.

This page contains links to wildland-urban interface fire computer models and example input files. Validation of these models is ongoing but far from complete. Please keep this in mind when using the Wildland-Urban Fire Dynamics Simulator (WFDS) to solve for problems outside the current scope of validation. See below for information on the current state of validation.

In general, please use the FDS discussion group and issue tracker when you have questions, and refer to WFDS. You may also email Ruddy Mell at wmell@comcast.net

Dateline: November 8, 2010

CAUTION: These Models are in the Early Stages of Validation

This page contains links to the National Institute of Standards and Technology (NIST) wildland-urban interface fire computer models and example input files. Validation of these models is ongoing but far from complete. Please keep this in mind when using the Wildland-Urban Fire Dynamics Simulator (WFDS) for problems outside the current scope of validation. See download sections below for information on the current state of validation.

In general, please use the FDS discussion group and issue tracker when you have questions, and refer to WFDS. You may also email Ruddy Mell at wmell@comcast.net

New users, please send an e-mail to Ruddy Mell so that he can inform you when new versions, capabilities, and a user guide are available.

II. WFDS User Information

This discussion is weighted toward Windows operating systems

New Users

If you are a new users, please send an e-mail to Ruddy Mell (wmell@comcast.net) so that he can inform you when new versions, capabilities, and a user guide are available.

Installing WFDS: New users of WFDS will need to first install and perform simple runs with FDS and the companion visualization tool Smokeview (also developed at NIST). See http://www.fire.nist.gov/fds/downloads.html to obtain the self-extracting installation for FDS, Smokeview, sample FDS input files, and documentation.

Running WFDS and viewing its output is the same as for FDS. Users are encouraged to read the FDS and Smokeview user guides. A very basic draft user guide for WFDS, which assumes some knowledge of running FDS, is available here. Once FDS is installed then the WFDS program can be run by using the WFDS executable provided below. Sample input files containing vegetation are also below. See the FDS User Guide and the section for experiences users (below).

Input files. Sample input files are given below in the download sections. The quantities that define the vegetation are straightforward to understand. Some discussion of these are in the download sections. In general, users should define the thermophysical parameters of the vegetation with values that are similar to what's in the input files. If you have questions contact me at wmell@comcast.net.

Experienced Users

Running WFDS is identical to running FDS because WFDS capabilities are contained within FDS . The visualization package from NIST, called Smokeview, also works for WFDS. If you already have FDS installed, then download the appropriate WFDS executable (links are below) and use a sample input file from below. The program is run by typing (from within a command window for the MS Windows users)

wfds... input....

where:

"wfds..." is the WFDS executable obtain from the links below or compiled from the source (also from below), and

"input...: is the input filename.

In the example above it's assumed the FDS program resides somewhere in the path. You can set up your own path to the WFDS program. Or you can download the executable into C:\Program Files\FDS\FDS5\bin which is the default directory set up by the FDS self extracting set up program. If the program is in the same directory as the input file type

./wfds... input....

For use with multiple processors (assuming your computer has been configured correctly - see the FDS user guide) the command would be (this is standard for Windows)

mpiexec -n #proc wfds5... input...

for Windows, where #proc is the number of processors and wfds5 is the mpi enabled executable

III. Status of WFDS Model Development

There are currently two ways of representing vegetative fuel in WFDS:

Fuel element model. This model is fully integrated into FDS (links to the executable are also given below). In this approach the vegetation occupies a specified volume (e.g., trees crowns). In general, this model is to be used when the grid resolution can span the vegetation with a number of grid cells. Realistic fire spread through grass, or other surface fuels, can be simulated if the computational grid is sufficiently fine.

Boundary fuel model. This model has been incorporated into FDS5 but is undergoing testing. It is limited to surface fuels (e.g., ground cover, grass). Links to the out-dated version, more fully tested version but based on FDS4, are below.

As mentioned above, the procedure for running WFDS is the same as for FDS (only some of the inputs are different as can be seen in the sample input files below). Consult the FDS User Manual and the discussion group if you are having trouble and Ruddy is not available.

Validation Studies

Current Version

The most current version of WFDS for Windows, Linux, and OSX (built on November 1, 2010) is within the current version of FDS release (5.5.3), which can be downloaded (along with the most current version of the visualization program Smokeview) at http://code.google.com/p/fds-smv/downloads/list

Older Versions

Input Files using Fuel Element Model

Vegetation can be described by a rectangular, conical, cylindrical volume, or a frustum. In the input files below you'll find surface fuels described by a rectangular volume (e.g., pine needles), crown fuels as conical volumes, and tree stems as cylindrical volumes. The stems are present to serve as wind breaks and do not burn.

If you are including stems, please following the format of the input files.

Smokeview image on left shows particles that represent the surface and tree crown vegetation. Particles are colored according to their temperature value. A vertical slice file with colors showing the gas-phase temperature is also displayed.

Smokeview image on the right uses a solid object to display the cone shaped crown. The solid object is shown by default. This object can be removed in via the Smokeview commands:
Show/Hide - Geometry - Objects - Canopy.

About 90 identical 6 m tall, 3 m wide, cone shaped trees are randomly distributed across a 30 m wide and 25 m long area. Axes units are meters. A 0.5 m tall grass is underneath the trees. The grass is ignited on the upwind side. The wind speed is uo = 2 m/s at the x=0 plane.
Grid resolution is 0.5 m in all directions. Note that, unlike the more resolved (~7.5 cm grids) isolated tree simulations, validation of the fuel element model at this resolution has not been completed. The simulation time is 60 s and requires about 25 cpu minutes for a single 3.8 GHz processor run and about About 200 MB of memory is required. The trees in the above image are colored according to the temperature of the crown vegetation.

Fire, trees, and smoke plume. Same domain size and tree definition as above. Axes units are meters. Fire is held stationary with bed dimensions of 2 m deep by 30 m long with a heat release rate of 500 k/m^2 (based on Australian grassland fires). Simulation time is 30 s which requires about 9 cpu minutes on a 3.8 GHz processor. About 200 MB of memory is required.

the user specifications in HEAD and TREE namelists will result in the output of following columns of data in a text (.csv) file:

Time (s)

Dry mass of vegetation (kg)

Moisture mass vegetation (kg)

Total net convective heat transfer (kW) summed over all fuel elements in the particular &TREE . This measure is the sum of the volume integral of the divergence of the convective heat flux on the fuel element. In other words it is the sum_i integral( divergence of convective heat flux) dVe; where Ve is the volume of the fuel element, and it is the index of the fuel element running from 1 to the number of fuel elements in the computational cell. This is, of course, computed indirectly, the sum is not actually carried out.

Total net radiative heat transfer (kW) summed over all fuel elements in the particular &TREE line (i.e., the same computation as was done for the covective heat flux).

The file name will be 'Test_Tree_1_foliage_vegout.csv'. The 'Test' part of the file name is obtained from CHID in the HEAD namelist and the 'Tree_1_foliage' part of the file name is from LABEL in the TREE namelist. The parameter OUTPUT_TREE must be set to .TRUE. in the TREE namelist.

In addition, the user can specify the time interval between data dumps to the file by the usual use of the DUMP namelist. For example,

&DUMP DT_VEG=0.1 /

will output the dry and moisture mass every 0.1 seconds.

V. WFDS for Raised Fuels Using the Boundary Fuel Model and the Current Version of FDS

The boundary fuel model is now (as of May 10, 2010) integrated with the current version of FDS (v. 5.5.0). It is still undergoing some testing so use with care and testing. It is obtained by downloading WFDS from above . A sample input file is given below. NOTE: the boundary fuel model is undergoing validation tests and should be used with this in consideration.

Validation Studies

Simulation of Australian grassland fires--This paper describes the modeling approach used to represent the surface vegetation. The validation studies in this paper are being repeated for the current version of WFDS.

Program Download

The same program is used for the boundary fuel and the fuel element vegetation models. Downloads and source code are in the section above.

Input Files

Example of fire spread up a simple hill. Fuel properties are based on Australian grassland fuel.

WFDS for surface fuels such as grassland fuels (boundary fuel model)
All links below are for a version based on FDS4 and are no loger supported. The boundary fuel model is now in the current version of WFDS as described in the section above.

<>LINUX multiple processor mpi executable wfdsp1_linuxmpi_bndryfuel.exe (3.3 MB)
run by typing mpirun ## wfds_linux_5-19-05_fds4p04_mpi.exe >& out & where ## defines the number of processors. NOTE you must have a file named "fds.data" in the same directory as the executable, this file contains only the name of the input file that WFDS is supposed to read. <><>

Input files with boundary fuel model
Vegetation is present on the bottom of the computational domain as a surface fuel. It can be thought of as being "painted" on. Note that input variable names differ from those in the fuel element model above. Both models will have idential input variable name when then are incorporated into FDS5.

Fire perimeter and smoke plume. The domain is split between the two processors as bordered by the colored lines. Same fuel parameters, domain size, grid resolution as case above (input_fireC064_1grid.txt) but runs on two processors. Axes units are meters. Required 5.6 cpu hours on dual 3.8 GHz processor computer. For details on running with multiple processors see the FDS user guide.

Fire line and smoke plume is shown. Domain in 4 m (x) by 1.25 m (y) by 2.2 m (z) with 80 (x) by 25 (y) by 45 (z) grid cells. Horizontal grid resolution is 5 cm; vertical starts at 2.5 cm at bottom and stretches to 20 cm at top. The fire spread from left to right in zero ambient wind. Green rectangle shows were the pine needle bed is. The color should change from green to something darker to denote what has been burned. This capability will be added to Smokeview. Simulation time of 300 s required about 3 hours of cpu time on a 3.8 GHz processor;66 MB of memory were required.